Prerequisites for MPLS LSP Ping and Traceroute for P2MP

Before you use the MPLS LSP Ping and Traceroute for P2MP feature, you should have the support for following:

Cisco IOS XR softwareRelease 4.0.0 or a later release

Configure Resource Reservation Protocol (RSVP) features on the headend, midpoint, and tailend routers
in the MPLS network

Configure traffic
engineering features on the headend, midpoint, and tailend routers
in the MPLS network

Enable MPLS OAM using the mpls oam command on all routers in the MPLS network

MPLS Network Management with MPLS LSP Ping and MPLS LSP Traceroute

To manage an MPLS network, you must have the ability to monitor LSPs and quickly isolate MPLS forwarding problems. You need ways to characterize the liveliness of an LSP and reliably detect when an LSP fails to deliver user traffic.

You can use MPLS LSP ping to verify the LSP that is used to transport packets. You can use MPLS LSP traceroute to trace LSPs that are used to carry packets destined for P2MP LSP.

An MPLS echo request is sent through an LSP to validate it. A TTL expiration or LSP breakage causes the transit router to process the echo request before it gets to the intended destination. The router returns an MPLS echo reply that contains an explanatory reply code to the originator of the echo request.

The successful echo request is processed at the egress of the LSP. The echo reply is sent through an IP path, an MPLS path, or a combination of both, back to the originator of the echo request.

Roles of Various Routers

A P2MP TE network contains the following elements:

Headend RouterThe headend router, also called the source or ingress router, is
responsible for initiating the signaling messages that set up the P2MP TE LSP. The headend
router can also be a branch point, which means the router performs
packet replication and the sub-LSPs split into different
directions.

Midpoint RouterThe midpoint router is where the sub-LSP signaling is processed.
The midpoint router can be a branch point.

Tailend RouterThe tailend router, also called the destination, egress, or
leaf-node router, is where sub-LSP signaling ends.
The router which is one of potentially many destinations of the
P2MP TE LSP.

Bud RouterA bud router is a midpoint and tailend router at the same time. An LSR that is an egress LSR, but also has one or more directly
connected downstream LSRs.

Branch RouterA branch router is either a midpoint or tailend router at any given time.

Transit RouterA transit router is an LSR that is not an egress router, but also has one or more directly
connected downstream routers.

A P2MP tunnel consists of one or more sub-LSPs.All sub-LSPs belonging
to the same P2MP tunnel employ the same constraints, protection
policies, and so on, which are configured at the headend router.

P2MP TE tunnels build on the features that exist in basic
point-to-point TE tunnels. The P2MP TE tunnels have the following
characteristics:

There is one source (headend) but more than one destination
(tailend).

They are unidirectional.

They are explicitly routed.

Multiple sub-LSPs connect the headend router to various tailend
routers.

P2MP Ping

The P2MP ping feature is used to check the connectivity between Ingress LSR and egress LSR, along a P2MP LSP. The Ingress LSR sends the P2MP echo request message along the specified P2MP LSP. All egress LSRs which receive the P2MP echo request message from the ingress LSR must send a P2MP echo reply message to the ingress LSR, according to the reply mode specified in the P2MP echo request message.

MPLS LSP ping uses MPLS echo request and reply packets to validate an LSP. You can use MPLS LSP ping to validate RSVP P2MP IPv4 FECs by using appropriate keywords and arguments with the ping mpls command.

The MPLS echo request packet is sent to a target router through the use of the appropriate label stack associated with the LSP to be validated. Use of the label stack causes the packet to be forwarded over the LSP itself.

The destination IP address of the MPLS echo request packet is different from the address used to select the label stack. The destination IP address is defined as a 127.x.y.z/8 address. The 127.x.y.z/8 address prevents the IP packet from being IP switched to its destination, if the LSP is broken.

An MPLS echo reply is sent in response to an MPLS echo request. The reply is sent as an IP packet and it is forwarded using IP, MPLS, or a combination of both types of switching. The source address of the MPLS echo reply packet is an address obtained from the router generating the echo reply. The destination address is the source address of the router that originated the MPLS echo request packet.

The MPLS echo reply destination port is set to the echo request source port.

Note

Only P2MP TE LSP IPv4 is supported. If the Responder Identifier TLV is missing, the echo request requests information from all responder-ids.

Jitter

Jitter is used to reduce the load on the LSR where the ping is performed. By adding a jitter, the replying routers will space their reply time based on a random number between 0 and the jitter value, Jitter TLV, specified in the packet.

P2MP Traceroute

The P2MP traceroute feature is used to isolate the failure point of a P2MP LSP. It is used for hop-by-hop fault localization and path tracing. The traceroute feature relies on the expiration of the TTL of the packet that carries the echo request. When the P2MP echo request message hits a transit node, it checks the TTL and if it is expired, the packet is punted to the control plane, else the message is forwarded or replicated. If punted to the control plane, a reply message is build based on the contents of the request message.

Traceroute can be applied to all nodes in the P2MP tree. However, you can select a specific traceroute target through the P2MP Responder Identifier TLV. An entry in this TLV represents an responder-id or a transit node. This is only the case for P2MP TE LSPs.

Note

Only P2MP TE LSP IPv4 is supported. If the Responder Identifier TLV is missing, the echo request requests information from all responder-ids.

Jitter

Jitter is used to reduce the load on the LSR where the traceroute is performed. By adding a jitter, the replying routers will space their reply time based on a random number between 0 and the jitter value, , Jitter TLV, specified in the packet.

For more information about ping and traceroute commands, see MPLS OAM commands chapter in the Cisco IOS XR MPLS Command
Reference for the Cisco CRS Router.

Configure the Ping and Traceroute: Example

This section contains examples of the ping and traceroute commands, based on this topology.

This example shows multiple destinations set on the assigned LSP path.